1. Field of the Invention
The present invention relates to a light-emitting diode (LED) in which light emitted by a light-emitting element is reflected by a concave reflection surface, and then reflected light is radiated to the outside. The term “light-emitting element” used herein refers to an LED chip itself, and the term “light-emitting diode” refers to a resin package carrying such an LED chip or a light-emitting device at large including an optical system such as a lens system.
2. Description of the Related Art
There have been devised light-emitting diodes having a variety of structures. Of such various light-emitting diodes, a reflection type light-emitting diode has a feature of being able to effectively radiate light emitted from a light-emitting element to the outside, and to be made thin. Description will be given below of the conventional reflection type light-emitting diode.
FIG. 27 is a schematic cross-sectional view of a first reflection type light-emitting diode according to the related art. The light emitting diode 80 shown in FIG. 27 comprises a light-emitting element 81, lead assemblies 82a and 82b, a bonding wire 83, a light-transmissible material 84, a reflection substrate 85 and a radiation plate 86.
The light-emitting element 81 is mounted on one end of the lead assembly 82a, and the light-emitting element 81 and the lead assembly 82b are electrically interconnected by the bonding wire 83. The refection substrate 85 is produced through molding of a resin and has a concave mirror surface 85a for light reflection on its center. The reflection mirror surface 85a has been formed on the concave surface of the reflection substrate 85 by subjecting the surface to an appropriate treatment, such as metal vapor deposition. Lead assemblies 82a and 82b are placed above the reflection substrate 85 so as to dispose the light-emitting element 81 opposite to the radiation plate 86. The radiation plate 86 is placed on the whole assembly. The radiation plate 86 is to transmit light reflected by the reflection mirror surface 85a and then to radiate it to the outside. Provision of the radiation plate 86 allows the high precision of the surface through which light is emitted outward. The space between the reflection mirror surface 85a and the radiation plate 86 is filled with a resin 84, and thus the light-emitting diode 81 is embedded in the resin.
FIG. 28 is a schematic cross-sectional view of a second reflection type light-emitting diode according to the related art. The light-emitting diode 90 shown in FIG. 28 comprises a light-emitting element 91, lead assemblies 92a and 92b, a bonding wire 93, a light-transmissible material 94, a concave reflection surface 95, a radiation surface 96, and resin-coated portions 97.
The light-emitting element 91 is mounted on one end of the lead assembly 92a, and the light-emitting element 91 and the lead assembly 92b are electrically interconnected by the bonding wire 93. The light-emitting element 91 and the tip end portions of the lead assemblies 92a, 92b and the bonding wire 93 are integrally sealed by the light-transmissible material 94. The concave reflection surface 95 is produced by mirror-surface-treating one surface of the light-transmissible material 94 according to a suitable method such as plating or metal vapor deposition, and is formed on the side opposite to the light-emitting surface of the light-emitting element 91. A planar radiation surface 96 is formed on the light-transmissible material 94 at its surface opposite to the concave reflection surface 95.
The resin-coated portion 97 is a portion at which a part of lead 92a or 92b is coated with a light-transmissible resin, and is formed around the circumference of the concave reflection surface 95. The resin-coated portion 97 protrudes towards the side at which the concave reflection mirror resides. The reason why the resin-coated portion 97 is implemented is as follows. Assume, as an illustration, that the concave reflection mirror 95 be formed by metal vapor deposition. One surface of the light-transmissible material 94 upon which the mirror is to be formed is totally masked through deposition of an inert vapor excepting the site at which the mirror is to be formed, and then the exposed site receives metal vapor deposition to therewith form a mirror. In this manner, the concave reflection mirror surface 95 is produced. However, if the resin-coated portion 97 were not implemented, metal vapor might creep beneath the mask during metal vapor deposition, adhere to parts of lead assemblies 92a and 92b, and shunt the leads. To avoid the problem involved in the shunting of lead assemblies 92a and 92b, the resin-coated portion 97 is implemented.
The width w of the resin-coated portion 97 has a certain limitation. The lead assemblies 92a and 92b are obtained by removing unnecessary parts from a lead frame. They are prepared by so-called trimming whereby unnecessary parts are cut away from a lead frame, or some other parts are folded at predetermined positions to receive lead assemblies 92a and 92b. If the resin-coated portion 97 had a too small width, parts of the resin-coated portion 97 would be torn off during trimming. To avoid this, it is necessary for the resin-coated portion 97 to have a width so large as to allow it to withstand trimming. To be specific, the width of the resin-coated portion 97 should be at least 1.0 mm.
As a method of manufacturing the reflection type light-emitting diode, there is used transfer molding in which a lead frame is held by upper and lower molding dies, and a thermosetting resin is injected between the molding dies and hardened. The reason why the transfer molding is used is that the reflection type light-emitting diode needs the reflection surface and the radiation surface precisely formed on both sides of the lead frame. The conventional production method based on transfer molding can easily yield light-emitting diodes in a mass production manner.
However, during production of the first light-emitting diode 80 according to the prior art, it is extremely difficult to uniformly fill the concave cavity above the reflection substrate 85 with a resin 84. This is partly because there is no proper apparatus known in the prior art that achieves the required filling of a resin. An alternative conventional method consists of putting an appropriate amount of resin into the concave cavity above the reflection substrate 85, and then placing the lead assemblies 82a and 82b and the radiation plate 86 above the reflection substrate 85, thereby producing a light-emitting diode 80. With this method, however, bubbles may easily creep into the gap between the reflection mirror surface 85a and the radiation plate 86, or the resin 84 may spill over the reflection substrate 85 onto its sides. Bubbles, if any, will affect the radiation characteristics of the light-emitting diode 80, and resin spills, if any, will require an additional complicated technique for their removal. As discussed above, the first light-emitting diode according to the prior art is problematic because its production is difficult and its amenability to mass production is limited.
On the contrary, the second light-emitting diode according to the prior art, although it advantageously allows mass production, is problematic because it can not be produced by a process based on the use of a reflow furnace. Specifically, if wiring of such light-emitting diodes is performed using paste-solder contained in a reflow furnace, the metal coat to serve as the reflection surface 95 might be torn off from the sealing resin 94 because the expansion coefficients of the metal coat and the sealing resin are quite different. If such tears occurred, creases would develop on the reflection surface 95, and would damage the function of reflection surface which depends on the proper reflection of light emitted by light-emitting element 91.
The second light-emitting diode 90 according to the prior art has another problem. With the diode in question, the space between the edge of the reflection surface 95 and the lead assembly 92a or 92b is so narrow that it easily develops cracks when the lead frame is trimmed. The narrowness of the space in question is ascribed to the broadness of the angle with which the light-emitting element views the opposite ends of the reflection surface 95. If that space remains too narrow, it will easily develop cracks during trimming regardless of the transverse width of the resin-coated portion 97.
The second light-emitting diode according to the prior art poses a still other problem: the transverse width w of the resin-coated portion 97 has a lower limit. Because of this, if such light-emitting diodes 90 are arrayed into a grid pattern, the distance between the adjacent diode must be two times the sum of the radius of the circle represented by the circumference of reflection surface 95, the transverse width of resin-coated portion 97, and the distance from the outer edge of resin-coated portion 97 to the angle at which lead assembly 92a or 92b is bent. This makes the light-emitting diode 90 inadequate to be densely packed into a grid pattern.
The second light-emitting diode according to the prior art poses a still other problem. The resin-coated portion 97 is so configured as to easily capture bubbles during molding. Even if molding conditions were adjusted to prevent this, the configuration of resin-coated portion 97 would inevitably lead to the development of more or less voids because the quality of products in a lot is subject to more or less variations. Thus, the implementation of the resin-coated portion 97 may act as a factor responsible for lowering the yield of products.